Time Machine not seeing internal iMac HD

[brainwave89]

I've setup my 2015 2013 iMac to boot of off an external SSD. I want to use the internal iMac HD to be used as my Time Machine backup. The iMac is running Catalina. Time Machine does not see the internal hard drive as an option.

What can I do so Time Machine can see my internal iMac hard drive?

Update: I choose to view all devices in Disk Utility and then erased that drive. That solved the issue.

Thank you for looking.

 

[chscag]

Your internal hard drive is formatted to APFS, Time Machine in its current iteration can only work with drives formatted to HFS+. Since you're booting from an external SSD, instead of using Time Machine for backups, use Carbon Copy Cloner or SuperDuper instead to make a bootable clone.